This invention relates to a method for promoting the photooxidative destruction of organic wastes and, more particularly, to a method utilizing particles of a biodegradable gelled metal alginate which have been stained with a sensitizing dye.
In recent years a justifiable concern has arisen over the presence in water supplies of organic waste pollutants. Among the more toxic constituents of effluents are phenols, cresols, xylenols, polyphenols, etc., which are introduced into the environment from chemical, petrochemical and plastics industries, refineries and coke ovens, and even from decaying vegetation. In a recent report to the United Nations Food and Agriculture Organization (Water Res.,7, 929-41 (1973)), the limit on combined phenols was recommended to be 1 ppm to ensure survival of salmonoids and 2 ppm for coarse fish.
The problem is compounded when water is treated with chlorine for municipal use. Chlorine reacts with phenols in the ppm range to produce o- and p-chlorophenols up to 2,4,6-trichlorophenol. Because of the off-taste and the odor arising from p-chlorophenol in water, the U.S. Public Health Service (1962) suggested a limit of 0.001 ppm of phenol in drinking water. This is lower than that for cyanide and arsenic. Toxicity of these phenols to fish is also greater than that of the unchlorinated phenols.
An outline of these problems and some of the current solutions is given in J. E. Zajic, "Water Pollution, Disposal and Reuse", Vol. 2, pp. 418-426 (1971). There, Zajic reports that the following treatments for phenolic wastes may be used: (1) solvent extraction, (2) steam stripping, (3) adsorption, (4) ion exchange, (5) chemical degradation and (6) biological degradation.
If the phenol concentration is high enough, recovery by solvent extraction or steam stripping may be economically feasible. However, phenol concentrations in most industrial effluents are not sufficiently high to justify such procedures. For this reason, phenols in dilute industrial effluents are usually adsorbed on activated carbon, from which they may be recovered. On the other hand, Zajic lists several limitations to this method, viz: lack of continuity in the process and contamination of the activated carbon by tars and tar acids beyond regeneration.
Similarly, because of their toxicity to most microorganisms, biodegradation of phenols is slow; however, the products are non-toxic, and this is a feasible method of treatment if the concentration is not too high. Chemical oxidation by ozone, chlorine or potassium permanganate is the fastest degradative treatment, but the cost in power and chemicals is high. In addition, as Zajic notes, with elemental chlorine precaution must be taken to ensure complete reaction of the phenol or the partially chlorinated phenol derivatives impart objectionable taste and toxicity to the water.
For these reasons, photooxidation has been studied as a possible solution to the problem. It is attractive because instead of requiring stoichiometric quantities of electrically or electronically produced oxidants, such as chlorine or ozone, the reaction takes place in the presence of sunlight, air and catalytic quantities of a promoter. Zinc titanate, zinc oxide, titanium dioxide and beach sand have been found to promote photocatalytic oxidation of dissolved organic matter on irradiation with sunlamps. See Kinney et al., "Photolysis Mechanisms for Pollution Abatement", Report No. TWRC-13, U.S. Dept. of Interior, Federal Water Pollution Control Adm., Ohio Basin Region, Cincinnati, Ohio, October 1969.
Dye sensitizers have also been used. Thus, Sargent and Sanks in a recent presentation at the Photochemical Reaction Engineering Symposium of the American Institute of Chemical Engineers in Washington, D.C., Dec. 1-5, 1974, entitled "Dye Catalyzed Oxidation of Refractory Organic Wastes Using Visible Light Energy" explained that the reaction sequence for dye sensitization involves absorption of light energy by the dye, transfer of dye energy to dissolved oxygen to form energized oxygen, and oxidation of the organic waste by the energized oxygen.
Accordingly, Sargent and Sanks describe the use of dyes in a homogeneous solution to promote aerobic photooxidation of organic wastes. While this system suffers from the instability of the dye in the homogeneous phase and the need to stain large volumes of water effectively, Sargent and Sanks find it preferable to the resin-bound dyes which they also tested since binding to ion exchange resins was found to reduce the effectiveness of the dye as a sensitizer and slow down the reaction as compared to dissolved dyes.
Still, it would be desirable for the sensitizing dye to be bound to a particulate material since treatment with homogeneous phase dyes results in an intermixing of the dye and the water treated which may be objectionable; whereas, with stained solid particles the phases remain separate. Similarly, use of heterogeneous phase dye-stained particles permits easy recovery of the stained particles by sedimentation or screening, allowing for reclaiming, restoring or reusing. It would also be particularly desirable if a biodegradable base material could be used since this would allow for distribution of the stained particles in polluted lakes and ponds in a one-time application without any detrimental environmental effect.
In regard to resin-bound dye sensitizers, it is noted that Blossey, Neckers, Thayer and Schaap in an article entitled "Polymer-Base Sensitizers for Photooxidants" in the Journal of the American Chemical Society, 95:5820 (1973) report on the use of a Rose Bengal polymer-based reagent to sensitize the generation of singlet molecular oxygen for promoting photochemical reactions. The base used was an insoluble styrene-divinylbenzene copolymer bead. This material, however, is not biodegradable and would be difficult to regenerate.
Accordingly, the need still exists for an effective polymer-based dye sensitizer for promoting photooxidative destruction of organic waste, especially a material which would be biodegradable while at the same time stabilizing the dyes to prolong their useful life over that which they evidence in homogeneous phase systems.